The molecular stuffing method is known as a process for producing a glass product having a refractive index gradient. In accordance with this method, a dopant (a refractive index correction component) is allowed to fill micropores of a porous glass product in such a manner that a dopant concentration gradient is formed in the porous glass product and the micropores are collapsed by applying a heat treatment (calcination). U.S. Pat. No. 4,110,093 discloses a process for producing a glass product having a refractive index gradient which comprises permeating a dopant solution in the porous glass product, leaching out a part of the dopant from the porous glass product to form a concentration gradient of the dopant distributed in the micropores of the porous glass product, precipitating the dopant in the micropores, drying the glass product, and then subjecting to a heat treatment to collapse the micropores.
It is also known that of dopants which can be used in the molecular stuffing method, a thallium compound such as thallium nitrate or thallium nitrite gives a relatively large refractive index gradient.
Glass products having a refractive index gradient which are used as rod-shaped microlenses for microlens arrays and microlenses coupling an optical communication fiber and a light source are required to be optically transparent. In producing glass products of this type by the molecular stuffing method, it is necessary to avoid the use of dopants which cause defects responsible for light scattering, viz., dopants which cause a phase separation in the glass product or precipitate crystals in the glass product. In fact, when rod-shaped glass products having a refractive index gradient are produced using thallium compounds which are known to provide a relatively large refractive index gradient, by the molecular stuffing method disclosed in U.S. Pat. No. 4,110,093 described above, bluish white turbidity is undesirably formed in the central portion of the glass product.